Increased interest in the global environmental issues has led to a demand for superior fuel consumption performance in pneumatic tires, along with superior steering stability. Techniques involving increasing tire rigidity for the purpose of enhancing steering stability are known. However, when the amount of a filler, compounded in a rubber composition used to form a tread portion, is increased for the purpose of increasing the rigidity of a tire, fuel consumption performance declines. Therefore, a large amount of silica is compounded in rubber compositions for use in treads in order to suppress heat build-up and reduce rolling resistance, which leads to the improvement in the fuel consumption performance of the pneumatic tires. Additionally, improvement effects in grip performance (wet grip performance) on wet road surfaces are obtained by compounding silica.
However, silica has poor affinity with diene rubber and dispersibility tends to be insufficient. There are problems in these cases in that the effects of improving the dynamic visco-elasticity characteristics such as the loss tangent (tan δ) of the rubber composition and the like have not been achievable. Additionally, reinforcing effects tend to be lower when compounding silica in a rubber component than when compounding carbon black. As a result, there is a problem in that when dispersibility is poor, wear resistance is insufficient.
To resolve this problem, Japanese Unexamined Patent Application Publication No. 2009-091498, WO/2005/021637 and WO/2003/102053 propose improving the dispersibility of silica by compounding silica in a rubber composition with a terminal-modified solution polymerization styrene butadiene rubber where the terminals are modified by a polyorganosiloxane or the like, thereby reducing heat build-up (tan δ at 60° C.). While effects of reducing heat build-up can be obtained with such a rubber composition, users demand even greater enhancements in low rolling resistance and steering stability. Thus, there is a need for greater improvement in low rolling resistance and steering stability.